While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration densities and operating speeds in LSI devices, DUV and EUV lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is thought requisite to the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. Although lithography using F2 laser (157 nm) was initially thought promising as the next lithography for 45-nm node devices, its development was retarded by several problems. A highlight was suddenly placed on the ArF immersion lithography that introduces a liquid having a higher refractive index than air (e.g., water, ethylene glycol, glycerol) between the projection lens and the wafer, allowing the projection lens to be designed to a numerical aperture (NA) of 1.0 or higher and achieving a higher resolution. While the ArF immersion lithography has entered the commercial stage, the technology still needs a resist material which is substantially non-leachable in water.
In the ArF lithography (193 nm), a high sensitivity resist material capable of achieving a high resolution at a small dose of exposure is needed to prevent the degradation of precise and expensive optical system materials. Among several measures for providing high sensitivity resist material, the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, polyacrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene, ring-opening metathesis polymerization (ROMP) polymers, and hydrogenated ROMP polymers have been proposed as the base resin. This choice is effective to some extent in enhancing the transparency of a resin alone.
With the rapid progress toward miniaturization, it becomes difficult to form a pattern of desired size from a resist material. In particular, the influence of acid diffusion is detrimental to lithography performance. As the pattern size is approaching the diffusion length of acid, the degradation of contrast becomes more serious. As the mask error factor (MEF), indicative of a dimensional shift on wafer relative to a dimensional shift on mask, increases, a noticeable drop of mask fidelity ensues. Further there is the tendency that the depth of focus becomes shallower as the wavelength of the light source becomes shorter. It is thus desired that the resist material have a depth of focus to enable resolution over a wide imaging range even when a light source of short wavelength is used. In addition, the fluctuation of pattern line width, known as line width roughness (LWR), becomes a problem. In the step of processing gate electrodes in the LSI circuit manufacturing process, for example, poor LWR gives rise to such problems as leak current, degrading electrical properties of transistors.
Accordingly, to take full advantage of wavelength reduction of the light source and increase of NA, it is ideal for a resist material that the acid generated therein be limited to exposed regions and uniformly distributed therein. It is thus necessary to control acid diffusion at a higher level than in prior art resist materials.
In addition to the above problems, the immersion lithography suffers from problems including a failure of resist pattern profile caused by defects resulting from microscopic water droplets left on the resist-coated wafer after exposure, and collapse or T-top configuration of resist pattern after development. For the immersion lithography as well, a pattern forming process capable of forming a satisfactory resist pattern after development is desired.
To solve the outstanding problems, studies are made not only on base resins and photoacid generators, but also on diffusion controlling agents. Amines are typically used as the diffusion controlling agent. Many problems associated with line width roughness (LWR) as an index of pattern roughness and pattern profile are left unsolved. Also use of weak acid onium salts as the diffusion controlling agent is under study. For example, Patent Document 1 describes a positive photosensitive composition for ArF excimer laser lithography comprising a carboxylic acid onium salt. The composition is based on the mechanism that a salt exchange occurs between a weak acid onium salt and a strong acid (sulfonic acid) generated by a PAG upon exposure, to form a weak acid and a strong acid onium salt. That is, the strong acid (α,α-difluorosulfonic acid) having high acidity is replaced by a weak acid (alkanesulfonic acid or carboxylic acid), thereby suppressing acid-aided decomposition reaction of acid labile group and reducing or controlling the distance of acid diffusion. The onium salt apparently functions as a quencher, that is, diffusion controlling agent. However, as the microfabrication technology is currently further advanced, the resist compositions using such weak acid onium salts become unsatisfactory with respect to resolution, MEF, LWR and depth of focus, particularly when processed by the ArF immersion lithography.
Patent Documents 2 and 3 describe photodegradable bases in the form of a sulfonium salt whose anion moiety has incorporated therein a nitrogen-containing substituent group, adapted to be decomposed to lose their basicity upon exposure. However, resist materials comprising these bases still lack the required lithography properties for the current generation of ArF lithography and ArF immersion lithography for ultrafine size processing. There is an increasing need for an effective acid diffusion controlling agent.